A vehicle drum brake has an initial (designed in) clearance between the brake shoes and the drum. The drum brake typically has an automatic slack adjuster that expands the shoes to compensate, as this clearance increases with wear. A problem can occur if the drum expands substantially from heat, for example, when a heavily loaded vehicle traverses a long downhill stretch and repeated applications of the brakes are needed. In this case, the expansion of the drum increases substantially the clearance between the shoes and the drum, possibly leading to a decreased braking effect. The automatic slack adjuster can expand the shoes in an attempt to keep up with the expansion of the drum. When a drum that is hot later cools off, however, it can shrink back to original shoe to drum clearance, minus any automatic adjustment that occurred during braking on the hill. The drum can in some cases shrink enough to bind on expanded shoes, causing brake drag. Some manufacturers compensate for this possibility by limiting the amount of adjustment that is provided with each brake application, in other words, very small increments, to reduce the likelihood of dragging brakes.
Some known slack adjusters adjust on the apply stroke. These adjusters, however, do not start this adjustment at the beginning of the apply stroke. Also, they do not have an increased, designed in clearance.